Metal nanoparticles are defined as having an average particle size less than 100 nm, and they have high surface area and high reactivity. Metal nanoparticles are an increasingly important industrial material, finding use as reaction catalysis (including serving as a reaction substrate), improving the behavior and properties of materials, drug delivery, as catalysts for the synthesis of carbon nanotubes, as a catalyst for hydrogen gas synthesis, and in the production of metal hydrides.
Metal nanoparticles can be produced by several techniques such as plasma or laser-driven gas phase reactions, evaporation-condensation mechanisms, and various wet chemical techniques. For example, U.S. Pat. No. 6,660,680 to Hampden-Smith et al. discloses the use of metal organics as precursors for the production of electrocatalyst powders. In their process, an aerosol of droplets from a liquid containing the metal organic and the support material is generated, the droplet, suspended in a carrier gas, is heated to remove the liquid to form particles of the catalyst dispersed on the support phase. However, none of the current technique provides a reliable, simple, and low-cost method for production of nanoparticles of a controlled size. The current techniques produce particles with poor crystallinity, a wide distribution of sizes around a desired nanoparticle size, or require specialized equipment, long processing times, or expensive specialty chemicals. For example, the preferred methods for the production of metal nanoparticles are hydrometallurgy and spray pyrolysis. However, these methods have several major drawbacks including preparation and handling of toxic materials that are difficult to handle, environmental emission control requirements for gaseous and liquid effluents, and a difficulty to produce average particle sizes below 100 nm.
One potentially attractive wet chemical technique for synthesis of metal nanoparticles is thermal decomposition, as these reactions may be carried using relatively simple equipment. However, currently known methods of metal nanoparticle formation using thermal decomposition require addition of a separate surfactant, thus increasing the complexity and cost of the method.
A particular application of the metal nanoparticles is in the production of carbon nanotubes. Carbon nanotubes are hexagonal networks of carbon atoms forming seamless tubes with each end capped with half of a fullerene molecule. They were first reported in 1991 by Sumio Iijima who produced multi-layer concentric tubes or multi-walled carbon nanotubes by evaporating carbon in an arc discharge. In 1993, Iijima's group and an IBM team headed by Donald Bethune independently discovered that a single-wall nanotube could be made by vaporizing carbon together with a transition metal such as iron or cobalt in an arc generator (see Iijima et al. Nature 363:603 (1993); Bethune et al., Nature 363: 605 (1993) and U.S. Pat. No. 5,424,054). The original syntheses produced low yields of non-uniform nanotubes mixed with large amounts of soot and metal particles.
Presently, there are three main approaches for the synthesis of single- and multi-walled carbon nanotubes. These include the electric arc discharge of graphite rod (Journet et al. Nature 388: 756 (1997)), the laser ablation of carbon (Thess et al. Science 273: 483 (1996)), and the chemical vapor deposition of hydrocarbons (Ivanov et al. Chem. Phys. Lett 223: 329 (1994); Li et al. Science 274: 1701 (1996)). Multi-walled carbon nanotubes can be produced on a commercial scale by catalytic hydrocarbon cracking while single-walled carbon nanotubes are still produced on a gram scale.
Generally, single-walled carbon nanotubes are preferred over multi-walled carbon nanotubes because they have unique mechanical and electronic properties. Defects are less likely to occur in single-walled carbon nanotubes because multi-walled carbon nanotubes can survive occasional defects by forming bridges between unsaturated carbon valances, while single-walled carbon nanotubes have no neighboring walls to compensate for defects. Defect-free single-walled nanotubes are expected to have remarkable mechanical, electronic and magnetic properties that could be tunable by varying the diameter, number of concentric shells, and chirality of the tube.
It is well known that the diameter of the SWNTs produced is proportional to the size of the catalyst particle. It is generally recognized that smaller catalyst particles of less than 3 nm are preferred for the growth of smaller diameter carbon nanotubes. Further, the distribution of the catalyst particle size influences on the yield and homogeneity of the SWNTs. Thus, smaller the catalyst particle size and narrower the distribution of the particle sizes, the higher the quality of the SWNTs. However, the smaller catalyst particles easily aggregate at the higher temperatures required for the synthesis of carbon nanotubes. Further, catalysts of small average particle sizes are difficult to synthesize. For example, Tanaka et al. (2004) Carbon 24: 1285-1292, disclose a method of catalyst preparation that includes the precipitation of ferric and nickel carbonate from ferric nitrate and nickel nitrate solutions for the synthesis of carbon nanofibers. In addition, the ratio of catalyst to support determines the quantity and quality of the SWNTs produced. Typically, a ratio of catalyst to support of about 1:10 to about 1:20 is used. It is difficult to increase the catalyst load on the support because the catalyst particles aggregate. Consequently, it has been reported that catalyst to support ratios of 1:1 to 1:3 are not suitable for the production of SWNTs. However, increasing the ratio increases the cost of the materials for the bulk production of SWNTs.
Thus, there is a need for simple, reliable, and low cost thermal decomposition methods for producing supported metal nanoparticles having higher catalyst loads that allows for control of the average particle size while minimizing the amount of variance in the particle sizes. Accordingly, the present invention provides methods and processes for the production of metal nanoparticle catalysts that can be used for the bulk synthesis of SWNTs.